(1) Field of the Invention
The present invention relates to a tough acrylic sheet suitable for a glazing material and also to a compression forming method and apparatus for the production thereof.
(2) Description of the Prior Art
Acrylic sheets having a thickness of 1 to 10 mm, which are generally used at the present, are formed by extrusion molding of polymethyl methacrylate (hereinafter referred to as "PMMA") or by cell casting of methyl methacrylate (hereinafter referred to as "MMA"). These acrylic sheets are ordinarily excellent in the transparency, weatherability, hardness, and rigidity and are used as glazing materials for vehicles or buildings. However, they are poor in resistance to shock. Accordingly, development of acrylic sheets having an improved impact or shock resistance while retaining good weatherability, hardness and rigidity has been desired. A product obtained by copolymerizing an acrylic resin with a rubber component has been proposed as an acrylic sheet having an improved impact resistance. However, the copolymerization with a rubber component results in the reduction of the weatherability, hardness, and rigidity. Furthermore, biaxial orientation of an acrylic resin has been proposed. However, no satisfactory results can be obtained according to this proposal. Biaxially oriented PMMA sheets having a thickness of 1 to 10 mm, which have heretofore been marketed, are manufactured according to the stretching method disclosed, for example, in U.S. Pat. No. Re. 24,978. Biaxially oriented sheets according to the stretching method are ordinarily formed in the lowly viscous state, and therefore, the degree of the biaxial orientation is low and the orientation release stress (hereinafter referred to as "ORS") is small. Accordingly, an acrylic sheet having a sufficient toughness has not been provided according to the stretching method.
The properties of biaxially oriented PMMA sheets are described in detail in Kunststoff-Handbuch, Band IX, Polymethacrylate, pages 125-134 (Carl Hanser Verlag, Munchen, 1975). In this known literature reference, it is taught that the breaking energy of a biaxially oriented PMMA sheet is the greatest when the stretch ratio is 3, but if the stretch ratio exceeds 3, the breaking energy is reduced. Accordingly, it may be said that the stretch ratio of 3 is preferred in the industrial production. In fact, in commercially available biaxially oriented PMMA sheets, the stretch ratio is about 3.
In the conventional compression forming apparatus, a mold defining a mold cavity and a die plate of a clamping device are integrated with each other. When the mold is heated and cooled at one step in the compression forming process, the die plate is also heated and cooled. Accordingly, the time required for the completion of one step is long and the energy consumption is large.
U.S. Pat. No. 3,632,841 discloses a compression forming method in which an acrylic resin is biaxially oriented with the use of a lubricant. In this method, the compression pressure is relatively low and in the range of from 300 to 600 psi (21 to 42 kg/cm.sup.2). Accordingly, the stretching rate is low, and the stretch ratio is low and less than 3. When ORS indicating the degree of orientation was measured by us, it was found that the ORS of the sheet formed according to this known method was approximately 5 to 15 kg/cm.sup.2. Thus, it was confirmed that the degree of orientation in this acrylic sheet is low and the acrylic sheet exhibits a poor toughness. If the compression pressure is increased in the method of the above U.S. patent, the frictional force between the acrylic resin and the mold is increased proportionally to the compression pressure, and the lubricating property is undesirably degraded. In order to prevent the formation of cracks, in U.S. Pat. No. 3,632,841, it is specified that compression should be performed under a low compression pressure and molding should be carried out at a low stretching rate.
As is apparent from the foregoing description, a biaxially oriented PMMA sheet having a high degree of orientation, which is formed under a high compression pressure, has not been reported.
Japanese Unexamined Patent Publication (Kokai) No. 56-30809 discloses a biaxially oriented acrylic sheet having a thickness of at least 1 mm, which is formed by compression forming. This acrylic sheet is obtained by applying a lubricant to a synthetic resin plate blank having a thickness of at least 5 mm, preheating the blank at a temperature between the glass transition temperature and the melting point of the blank, placing the blank in a compression mold heated at a temperature of at least the glass transition temperature of the blank and compressing the blank to biaxially orient the blank. According to this method, however, it is impossible to sufficiently improve the impact resistance without degradation of the characteristic properties of PMMA. The resin blank used in the method disclosed in the above Japanese patent publication is prepared by extrusion molding, compression molding or injection molding, and therefore, the molecular weight of the resin is low. Namely, the resin blank-constituting polymer is not an ultra-high-molecular-weight polymer. In this sheet, as the molecular weight is high, the sheet is more readily oriented, and a high toughness cannot be attained. In any of the compression forming methods taught in the above-mentioned two references, it is impossible to carry out compression forming economically advantageously.
Means for heating and cooling dies are disclosed, for example, in U.S. Pat. No. 3,632,841. For example, the provision of holes on a die, through which heating steam and cooling water are circulated to heat and cool the die is shown. A long time and a large quantity of energy are necessary for heating or cooling a metal die having a considerable thickness. Furthermore, if heating or cooling holes are formed in the vicinity of the surface of the mold so as to promptly heat or cool the mold, the heating or cooling of the surface of the mold becomes uneven, resulting in the production of a non-uniform acrylic sheet. In the molding of a biaxially oriented sheet, the uniformity of the temperature over the entire surface of the mold is important. When the mold is cooled, this temperature uniformity is especially important. In order to uniformly cool the surface of the mold, it is necessary to form a cooling water hole at a point considerably apart from the surface of the mold. If a hole for heating steam or cooling water is formed at such a distant point, a large quantity of energy and a long time are required for heating or cooling.